In the production mechanical paper pulps, it has long been recognized that a higher yield can be obtained from a given amount of raw cellulosic fibrous material compared to chemical pulping processes. Mechanical pulping refers to refiner mechanical pulping (RMP), thermomechanical pulping (TMP), chemimechanical pulping (CMP), and chemithermomechanical pulping (CTMP) and other methods of producing high-yield pulps. In mechanical pulping, the chips are broken down into progressively smaller chips or pulp using a refiner or the like. Typically, a refiner includes a relatively movable grinding surfaces defining a grinding zone therebetween wherein chips are reduced to form pulp. These grinding surfaces, for example oppositely disposed discs or conical surfaces, are relatively rotated by an electric motor. In areas where electrical costs are high, the cost of operating the refiner can be prohibitive. For example, approximately 1,000 kWh per ton of pulp may be used per refining stage with approximately 2,000 kWh per ton produced pulp for the conventional two-stage refiner.
It has been theorized that the power consumption of a refiner can be significantly reduced by reducing the grinding frequency of the refiner. It has been suggested that this be done by reducing the refiner speed. However when the refiner speed is reduced, so is the capacity of the refiner to produce pulp, and the reduced pulp production in almost every instance be considered unacceptable from the commercial standpoint.
According to the present invention, it is possible to significantly reduce energy consumption of the refiner while not significantly adversely affecting refiner capacity (pulp production). This is accomplished according to the present invention by significantly reducing the grinding frequency of the refiner, while at the same time significantly increasing retention time and power amplitude.
As used in the present specification and claims, "grinding frequency" means the number of relative revolutions per second (rps) of the grinding surfaces (the number of rotor revolutions/sec. where there is a rotor and a stator) multiplied by the number of grooves (cutting elements) at the pulp discharge end of the rotor grinding surface. In conventional commercial diskrefiners, the rotor is rotated at between about 1,000-1,800 rpm, with the cutting elements commonly numbering between about 400-600, so that a grinding frequency of at least about 6,000 Hz is provided, and the grinding frequency can be 30,000 Hz, or even more. According to the present invention, the grinding frequency is reduced by an order of magnitude or two. Typically, according to the present invention the number of cutting elements and the rpms of the grinding surfaces are provided so that a major portion of the power dissipation of the refiner takes place at a grinding frequency of between about 200-2,000 Hz, preferably between about 300-900 Hz (e.g. between about 300-800 Hz).
Pulp production is maintained according to the present invention, while power consumption is greatly reduced, by significantly increasing both the retention time and the power amplitude. The retention time is the average amount of time fibrous cellulosic material is within the grinding zone, and the power amplitude is the edge bar load.
According to the present invention, the retention time is increased by significantly increasing the retention volume. The retention volume is increased by removing the majority of the steam (e.g. at least about 90% of the steam) in generally the area that it is generated within the grinding zone. The steam takes up a significant amount of space in the grinding zone, and if removed then the retention volume is increased greatly. Retention volume is further increased because a minimum number of cutting elements are utilized, and the cutting elements themselves take up volume within the grinding zone. For example, the number of cutting elements can be limited to between about 12-67 (e.g. 20-60) compared to about 400-600 in conventional refiners. Still further, the retention volume can be increased by the configuration of the grinding zone. The grinding zone can be configured in the shape of a volume of revolution (a cone or cylinder) by disposing the grinding surfaces so that they are frusto-conical or cylindrical. Retention time according to the invention is at least about a second, and typically is on the order of greater than three seconds, and with 90% steam removal could typically be expected to be in the range of five-six seconds. This compares with a retention time of about five milliseconds in conventional disk refiners. Thus the retention time is at least about 100 times greater according to the invention than in conventional disk refiners.
Power amplitude is inherently increased according to the invention when the number of bars are minimized, and the grinding zone is defined so that it is "longer", increasing the effective length of the cutting elements. The power amplitude according to the invention could typically be at least double that of a conventional disk refiner, and can be expected to be on the order of about five times greater.
The cellulose pulp produced utilizing the methods according to the invention will have different properties than conventional mechanical pulps since the net affect of the new procedures according to the invention will be to cause structural changes to the fibrous material in different ways than they have typically occurred in conventional disk refiners.
The design of the refiner according to the present invention, having a minimum number of cutting elements (bars) results in much more area of the grinding elements being available for steam removal so that the steam can be removed in an effective manner to achieve the desired increase in retention time. According to the invention, also, this additional area for steam removal is effectively utilized, and the steam velocity is controlled, so as to remove the majority (e.g. at least about 90%) of the generated steam while minimizing the amount of fiber withdrawn with the steam.
According to one aspect of the present invention, there is provided a method of refining cellulosic fibrous material utilizing juxtaposed relatively movable grinding surfaces defining a grinding zone between them, with a material inlet to the grinding zone and a material outlet from the grinding zone, comprising the steps of: (a) Grinding the material between the grinding surfaces so that the majority (if not all) of power dissipation of the refiner takes place at a grinding frequency of about 200-2,000 Hz (preferably 300-900 Hz, e.g. 300-800). And (b) retaining the material within the grinding zone at a retention time of at least about one second (preferably greater than three seconds, e.g. on the order of about five-six seconds).
According to another aspect of the present invention there is provided a method of refining cellulosic fibrous material utilizing juxtaposed grinding surfaces movable relative to each other with an inlet and outlet for the fibrous material, comprising the steps of: Grinding material between the surfaces at a grinding frequency of about 200-2,000 Hz; and removing the majority (e.g. at least about 90%) of the steam generated between the material inlet and outlet, approximately at the area of steam generation. Preferably there is also provided the step of forming the grinding surfaces so that a grinding zone between the grinding surfaces defines a volume of revolution (e.g. cone or cylinder) about an axis with the inlet and outlet adjacent opposite ends of the volume of revolution.
The invention also contemplates a method of refining cellulosic fibrous material having juxtaposed grinding surfaces capable of relative rotation, with an inlet and an outlet for the fibrous material, comprising the steps of significantly reducing energy consumption during refining, while maintaining substantially the same capacity, compared to a conventional commercial disk refiner having cutting elements numbering between about 400-600 and a grinding frequency of at least about 6,000. The method steps are accomplished by significantly reducing refiner frequency while significantly increasing (e.g. increasing 100 times or more) the retention time, and significantly increasing (e.g. at least doubling) the power amplitude.
The new cellulose pulp produced according to the present invention is difficult to define in terms of properties, but is clearly distinct from conventional mechanical pulps. Cellulose pulp according to the invention is best defined as pulp produced by practicing the method steps set forth above.
According to still another aspect of the present invention, a refiner is provided with structure for optimum removal of steam generated in the grinding zone, the structure according to the invention taking advantage of the minimum number of cutting bars, and thus the relatively large area on the grinding surfaces between the cutting bars.
It is the primary object of the present invention to provide for a significant reduction in power consumption of a refiner producing mechanical pulp, while maintaining refiner capacity (pulp production). This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.